Fix some build problems I ran into

1. Add `source="1.7"` to `build.xml` to because the source code requires Java 7 (e.g. `DeflaterOutputStream`).
2. Upgrade `build.xml` and `README.md` to use Bouncy Castle 1.48.
3. Change `build.xml`'s output directory from `build` to `bin` so `abe.sh`'s classpath will work correctly (and match Eclipse's `bin` output directory structure).

If you would like me to change or remove any of these commits, just let me know and I can split them into separate pull requests.

Thanks! Your android-backup-extractor was a lifesaver! :+1: